1. Field of the Invention
The present invention relates to electrographic imaging apparatus and methods and more particularly to improved devices and methods for transferring non-insulative toner to a transfer member of the kind having a high relative-conductivity.
2. Brief Description of the Prior Art
The most popular commercial form of electrographic imaging is the one in which electrostatic images, produced by exposure of a uniformly pre-charged photoconductor member, are developed with tribo-electrically-charged, electrically insulative toner particles. The developed images are, in turn, electrostatically transferred and fused to plain paper of the kind having a high relative-conductivity. In this procedure, the insulative toner is triboelectrically charged by a rubbing action with contiguous carrier particles, and this has presented problems in several regards. For example, the toner-to-carrier ratio of the developer mixture must be monitored and controlled precisely to achieve uniform density of development. Also, carrier particles of such developer mixtures become ineffective after extended use and must be replaced. Further, the carrier particles will abrade the imaging member if extreme care is not exercised. Of course, the carrier adds additional cost and complexity to the system.
In view of such problems with the carrier-toner approach, much effort has been devoted to designing a single-component (i.e. having only toner and no carrier) development system. Many single component developer approaches have been suggested; however, to this point only the single-component developer approach that has found substantial commercial usage is one using a toner that is electrically non-insulative and magnetic. In this approach a magnetic brush applies the toner particles to the electrostatic image, e.g., as disclosed in U.S. Pat. No. 3,816,840, and development occurs by an induced charge mechanism such as is explained in U.S. Pat. No. 3,166,432. There has been a significant limitation to the non-insulative toner approach, however. Specifically, non-insulative toner is extremely difficult to transfer to supports having high relative-conductivity, such as plain paper. The commercial implementations therefore have been with photoconductive coated paper, such as zinc oxide copy sheets, or with insulative coated papers, such as resin-coated copy sheets. Such coated copy sheets are not as desirable as plain paper from either the cost or esthetic viewpoints. Thus it would be highly advantageous to have a structural approach facilitating high quality electrographic imaging with non-insulative toner and transfer media of high relative-conductivity such as plain paper copy sheets.